Many, perhaps most physiological and pathophysiological processes can be controlled by the selective up or down regulation of gene expression. If methods existed for gene expression control, pathologies could be treated. Examples include the inappropriate expression of proinflamatory cytokines in rheumatoid arthritis, under expression of the hepatic LDL receptor in hypercholesteremia, over expression of proangiogenic factors and under expression of antiangiogenic factors in solid tumor growth, to name just a few. In addition, pathogenic organisms such as viruses, bacteria, fungi, and protozoa could be controlled by altering gene expression. There is a clear unmet need for therapeutic approaches that are simply able to up-regulate beneficial genes and down-regulate disease causing genes.
In addition to the direct therapeutic utility provided by the ability to manipulate gene expression, this ability can be used experimentally to determine the function of a gene of interest. One common existing method for experimentally determining the function of a newly discovered gene is to clone its cDNA into an expression vector driven by a strong promoter and measure the physiological consequence of its over-expression in a transfected cell. This method is labor intensive and does not address the physiological consequences of down-regulation of a target gene. Simple methods allowing the selective over and under-expression of uncharacterized genes would be of great utility to the scientific community. Methods that permit the regulation of genes in cell model systems, transgenic animals and transgenic plants would find widespread use in academic laboratories, pharmaceutical companies, genomics companies and in the biotechnology industry.
An additional use of tools permitting the manipulation of gene expression is in the production of commercially useful biological products. Cell lines, transgenic animals and transgenic plants could be engineered to over-express a useful protein product. The production of erythropoictin by such an engineered cell line serves as an example. Likewise, production from metabolic pathways might be altered or improved by the selective up or down-regulation of a gene encoding a crucial enzyme. An example of this is the production of plants with altered levels of fatty acid saturation.
Methods currently exist in the art, which allow one to alter the expression of a given gene, e.g., using ribozymes, antisense technology, small molecule regulators, over-expression of cDNA clones, and gene-knockouts. These methods have to date proven to be generally insufficient for many applications and typically have not demonstrated either high target efficacy or high specificity in vivo. For useful experimental results and therapeutic treatments, these characteristics are desired.
Gene expression is normally controlled through alterations in the function of sequence specific DNA binding proteins called transcription factors. These bind in the general proximity (although occasionally at great distances) of the point of transcription initiation of a gene. They act to influence the efficiency of formation or function of a transcription initiation complex at the promoter. Transcription factors can act in a positive fashion (transactivation) or in a negative fashion (transrepression).
Transcription factor function can be constitutive (always “on”) or conditional. Conditional function can be imparted on a transcription factor by a variety of means, but the majority of these regulatory mechanisms depend of the sequestering of the factor in the cytoplasm and the inducible release and subsequent nuclear translocation, DNA binding and transactivation (or repression). Examples of transcription factors that function this way include progesterone receptors, sterol response element binding proteins (SREBPs) and NF-kappa B. There are examples of transcription factors that respond to phosphorylation or small molecule ligands by altering their ability to bind their cognate DNA recognition sequence (Hou et al., Science 256:1701 (1994); Gossen & Bujard, PNAS 89:5547 (1992); Oligino et al., Gene Ther. 5:491–496 (1998); Wang et al., Gene Ther. 4:432–441 (1997); Neering et al., Blood 88:1147–1155 (1996); and Rendahl et al., Nat. Biotechnol. 16:757–761 (1998)). This mechanism is common in prokaryotes but somewhat less common in eukaryotes.
Zinc finger proteins (“ZFPs”) are proteins that bind to DNA, RNA and/or protein in a sequence-specific manner. Zinc fingers were first identified in the transcription factor TFIIIA from the oocytes of the African clawed toad, Xenopus laevis. ZFPs are widespread in eukaryotic cells. An exemplary motif characterizing one class of these proteins (C2H2 class) is —Cys—(X)2-4—Cys—(X)12—His—(X)3-5—His (where X is any amino acid). A single finger domain is about 30 amino acids in length and several structural studies have demonstrated that it contains an alpha helix containing the two invariant histidine residues coordinated through zinc with the two cysteines of a single beta turn. To date, over 10,000 zinc finger sequences have been identified in several thousand known or putative transcription factors. ZFPs are involved not only in DNA-recognition, but also in RNA binding and protein—protein binding. Current estimates are that this class of molecules will constitute about 2% of all human genes.
The X-ray crystal structure of Zif268, a three-finger domain from a murine transcription factor, has been solved in complex with its cognate DNA-sequence and shows that each finger can be superimposed on the next by a periodic rotation and translation of the finger along the main DNA axis. The structure suggests that each finger interacts independently with DNA over 3 base-pair intervals, with side-chains at positions −1, 2, 3 and 6 on each recognition helix making contacts with respective DNA triplet sub-site. The amino terminus of Zif268 is situated at the 3′ end of its DNA recognition subsite. Recent results have indicated that some zinc fingers can bind to a fourth base in a target segment (Isalan et al., PNAS 94:5617–5621 (1997). The fourth base is on the opposite strand from the other three bases recognized by zinc finger and complementary to the base immediately 3′ of the three base subsite.
The structure of the Zif268-DNA complex also suggested that the DNA sequence specificity of a ZFP might be altered by making amino acid substitutions at the four helix positions (−1, 2, 3 and 6) on a zinc finger recognition helix. Phage display experiments using zinc finger combinatorial libraries to test this observation were published in a series of papers in 1994 (Rebar et al., Science 263:671–673 (1994); Jamieson et al., Biochemistry 33:5689–5695 (1994); Choo et al., PNAS 91:11163–11167 (1994)). Combinatorial libraries were constructed with randomized side-chains in either the first or middle finger of Zif268 and then isolated with an altered Zif268 binding site in which the appropriate DNA sub-site was replaced by an altered DNA triplet. Correlation between the nature of introduced mutations and the resulting alteration in binding specificity gave rise to a partial set of substitution rules for rational design of ZFPs with altered binding specificity.
Greisman & Pabo, Science 275:657–661 (1997) discuss an elaboration of a phage display method in which each finger of a zinc finger protein is successively subjected to randomization and selection. This paper reported selection of ZFPs for a nuclear hormone response element, a p53 target site and a TATA box sequence.
Recombinant ZFPs have been reported to have the ability to regulate gene expression of transiently expressed reporter genes in cultured cells (see, e.g., Pomerantz et al., Science 267:93–96 (1995); Liu et al., PNAS 94:5525–5530 1997); and Beerli et al., PNAS 95:14628–14633 (1998)).
For example, Pomerantz et al., Science 267:93–96 (1995) report an attempt to design a novel DNA binding protein by fusing two fingers from Zif268 with a homeodomain from Oct-1. The hybrid protein was then fused with either a transcriptional activator or repressor domain for expression as a chimeric protein. The chimeric protein was reported to bind a target site representing a hybrid of the subsites of its two components. The authors then constructed a reporter vector containing a luciferase gene operably linked to a promoter and a hybrid site for the chimeric DNA binding protein in proximity to the promoter. The authors reported that their chimeric DNA binding protein could activate or repress expression of the luciferase gene.
Liu et al., PNAS 94:5525–5530 (1997) report forming a composite ZFP by using a peptide spacer to link two component ZFPs, each having three fingers. The composite protein was then further linked to transcriptional activation or repression domains. It was reported that the resulting chimeric protein bound to a target site formed from the target segments bound by the two component ZFPs. It was further reported that the chimeric ZFP could activate or repress transcription of a reporter gene when its target site was inserted into a reporter plasmid in proximity to a promoter operably linked to the reporter.
Beerli et al., PNAS 95:14628–14633 (1998) report construction of a chimeric six finger ZFP fused to either a KRAB, ERD, or SID transcriptional repressor domain, or the VP16 or VP64 transcriptional activation domain. This chimeric ZFP was designed to recognize an 18 bp target site in the 5′ untranslated region of the human erbB-2 gene. Using this construct, the authors of this study report both activation and repression of a transiently expressed reporter luciferase construct linked to the erbB-2 promoter.
In addition, a recombinant ZFP was reported to repress expression of an integrated plasmid construct encoding a bcr-abl oncogene (Choo et al., Nature 372:642–645 (1994)). The target segment to which the ZFPs bound was a nine base sequence GCA GAA GCC chosen to overlap the junction created by a specific oncogenic translocation fusing the genes encoding bcr and abl. The intention was that a ZFP specific to this target site would bind to the oncogene without binding to abl or bcr component genes. The authors used phage display to select a variant ZFP that bound to this target segment. the variant ZFP thus isolated was then reported to repress expression of a stably transfected bcr-abl construct in a cell line.
To date, these methods have focused on regulation of either transiently expressed genes, or on regulation of exogenous genes that have been integrated into the genome. The transiently expressed genes described by Pomerantz et al., Liu et al., and Beerli et al. are episomal and are not packaged into chromatin in the same manner as chromosomal genes. Moreover, even the stably expressed gene described by Choo et al. is randomly integrated into the genome and is not found in a native chromatin environment as compared to an endogenous gene.